In recent years, disc records in which recorded information is retrieved by reflection of a light beam have become popular. These optical disc records may be employed to store information, such as audio and/or video and data, in analog or digital format. Optical disc records include a metallic layer usually covered by a protective layer. Information is retrieved from an optical disc record by detecting light reflected from the metallic layer and decoding the reflected light to recover the information. Well known examples of optical disc records are "compact discs" and "video discs" on which audio and/or video information are recorded.
In two-sided optical disc records, a substantially equal thickness of a substantially transparent protective material is disposed on each side of the metallic layer. In a single-sided record, the protective material may be thicker on one side of the metallic layer than on the other. In that case, the protective material needs only to be transparent on the side of the metallic layer containing the recorded information.
Usually, the functions of playback apparatus for optical disc records are automated. For example, an optical disc record player may include a device for detecting the presence of a disc record. When a disc record is detected, the player automatically loads the record into its playback position. Likewise, the disc record player may not undertake to load a second record until its detector indicates a first record has been removed or until overriding manual instructions are provided.
A number of mechanical and optical devices for detecting the presence of an optical disc record in playback apparatus have been proposed. Mechanical detectors employ a physical contact to determine the presence of an optical disc record. Since optical disc records are susceptible to mechanical damage, the mechanical detectors can scratch the records, damage them, and interfere with information retrieval. The conventional optical detectors avoid a mechanical contact and usually function by sensing the reflection or interruption of a light beam by an optical disc record. These detectors employ infrared light and, as a result, are extremely sensitive to the precise position of the disc record relative to light beam sensors. Moreover, these optical detectors are relatively expensive, limiting their application, occupy a relatively large volume and must be precisely positioned for reliable performance.
Accordingly, it is desirable to provide a relatively low cost, dependable detector for detecting the presence and other characteristics of optical disc records.